This dataset contains initialised seasonal hindcast experiments for the 20th century, produced using a version of the ECMWF's Intergrated Forecasting System (IFS), and initialised from ECMWF reanalysis (ERA-20C/CERA-20C). Two types of hindcast experiment are included in this dataset:

CSF-20C (Coupled Seasonal Forecasts): This is a hindcast performed with the model fully coupled to the Nucleus for European Modelling of the Ocean (NEMO) ocean model, initialised from CERA-20C reanalysis data.

ASF-20C (Atmospheric Seasonal Forecasts): This is a hindcast performed with prescribed Sea Surface Temperature (SST) and sea-ice boundary conditions at the surface, initialised form ERA-20C reanalysis data.

Hindcasts were started every year between 1901-2009 (inclusive) on the 1st February, 1st May, 1st August and 1st November and are run out for four months from the start date. For each start date, there are either 51 or 25 ensemble members (depending on the season/hindcast).

Monthly mean values for the following variables have been archived here on CEDA (in alphabetical order):

LHF (latent heat-flux)

OLR (outgoing long-wave radiation)

PREC (precipitation)

SHF (sensible heat-flux)

SIC (sea-ice concentration)

SLP (sea-level pressure)

SST (sea surface temperature)

T2 (2m temperature)

U250 (zonal wind at 250 hPa level)

U850 (zonal wind at 850 hPa level)

Us (10m zonal wind speed)

V250 (meridional wind at 250 hPa level)

V850 (meridional wind at 850 hPa level)

Vs (10m meridional wind speed)

Z500 (geopotential at 500 hPa level)

Files for each ensemble member are archived separately and the filetree is in the following format:

[EXPname]/[VARname]monthly_[EXPname]_[INITmonth]START_ENSmems/[VARname]monthly_[YEAR]_M[ENSnum].nc

Where:

[EXPname] =CSF-20C or ASF-20C

[VARname] = Z500, Vs, V850, ... etc.

[INITmonth] = Feb, May, Aug or Nov

[YEAR] =1901, 1902, 1903, ... etc.

[ENSnum] =0, 1, 2, 3, ... etc.

More details of these hindcasts and some previous analysis of these datasets have appeared in the following papers:

Weisheimer, A., Schaller, N., O'Reilly, C., MacLeod, D. A., & Palmer, T. (2017). Atmospheric seasonal forecasts of the twentieth century: multi‐decadal variability in predictive skill of the winter North Atlantic Oscillation (NAO) and their potential value for extreme event attribution. Quarterly Journal of the Royal Meteorological Society, 143(703), 917-926.

O'Reilly, C. H., Heatley, J., MacLeod, D., Weisheimer, A., Palmer, T. N., Schaller, N., & Woollings, T. (2017). Variability in seasonal forecast skill of Northern Hemisphere winters over the twentieth century. Geophysical Research Letters, 44(11), 5729-5738.

MacLeod, D., O'Reilly, C., Palmer, T., & Weisheimer, A. (2018). Flow dependent ensemble spread in seasonal forecasts of the boreal winter extratropics. Atmospheric Science Letters, 19(5), e815.

Parker, T., Woollings, T., Weisheimer, A., O'Reilly, C., Baker, L., & Shaffrey, L. (2019). Seasonal predictability of the winter North Atlantic Oscillation from a jet stream perspective. Geophysical Research Letters, 46(16), 10159-10167.

Weisheimer, A., Decremer, D., MacLeod, D., O'Reilly, C., Stockdale, T. N., Johnson, S., & Palmer, T. N. (2019). How confident are predictability estimates of the winter North Atlantic Oscillation?. Quarterly Journal of the Royal Meteorological Society, 145, 140-159.

Weisheimer, A., Befort, D. J., MacLeod, D., Palmer, T., O’Reilly, C., & Strømmen, K. (2020). Seasonal forecasts of the 20th Century. Bulletin of the American Meteorological Society, (2020).